1) Field of the Invention
The present invention relates to a regular reflection output conversion method, a diffuse reflection output conversion method, and a toner amount-of-transfer conversion method, in transfer detection of toner such as toner, and an image forming apparatus such as a copying machine, a printer, a facsimile, and a plotter, capable of executing these methods, a toner transfer detection apparatus capable of executing these methods, and a gradation pattern used for these methods.
2) Description of the Related Art
Conventionally, in an image forming apparatus such as a copying machine and a laser beam printer using the electrophotographic method, a toner patch for density detection (hereinafter, “density pattern” or “density detection pattern”) is formed on an image carrier such as a photosensitive material, in order to obtain a stable image density at all times, the patch density is detected by an optical detecting unit, and based on the detection result, the development potential is changed (specifically, an LD power, a charging bias, and a development bias are changed).
In a case of a two-component development method, image density is controlled so that the maximum target transfer (a transfer for obtaining a target ID) becomes an intended value, by changing a target value for toner density control in a development unit.
For such a detecting unit for density detection patch, a reflecting type optical sensor including a light emitting diode and a photodetector is generally used. In the image forming apparatus, since a formed reference pattern is detected, the sensor is referred to as a P (pattern) sensor. Further, a light emitting diode (LED) is generally used for the light emitting diode for the P sensor, and a photodiode (PD) or a phototransistor (PTr) is generally used for the photodetector.
As the sensor configuration, there are three types, that is, (1) a type of detecting only regular reflection light, as illustrated in FIG. 14 (See for example, Japanese Patent Application Laid-Open No. 2001-324840), (2) a type of detecting only diffuse reflection light, as illustrated in FIG. 15 (See, for example, Japanese Patent Application Laid-Open No. H5-249787 and Japanese Patent Publication No. 3155555), and (3) a type of detecting both as illustrated in FIG. 16 (See, for example, Japanese Patent Application Laid-Open No. 2001-194843). Reference signs 250A, 250B, and 250C denote element holders, 251 denotes an LED, 252 denotes a regular reflection photodetector, 253 denotes a detection target surface, 254 denotes a toner patch on the detection target surface, and 255 denotes a diffuse reflection photodetector.
Recently, as illustrated in FIG. 17, a type in which a beam splitter is provided on the optical path on the light emission side and light reception side is also used frequently (4) (See, for example, Japanese Patent Publication No. 2729976 and Japanese Patent Application Laid-Open Nos. H10-221902 and 2002-72612). Reference sign 256 denotes an LED, 257 and 258 denote a beam splitter, 259 denotes a photodiode as a light receiving unit with respect to P-ray light (regular reflection light), and 260 denotes a photodiode as a light receiving unit with respect to S-ray light (diffuse reflection light).
A color image forming apparatus including one drum (photosensitive drum), revolver development, and an intermediate transfer body has been heretofore predominant. However, due to the recent trend of high speed and high function of the color image output unit, a so-called tandem-type color image forming apparatus becomes predominant recently, which has a configuration such that a plurality of imaging units (for example, units for four colors) including an image carrier, a development apparatus, and the like is arrayed opposite to a transfer belt, and toner images on the image carriers are sequentially transferred onto transfer paper (or a transfer belt).
In the image forming apparatus having a plurality of imaging units, arrangement of an optical detecting unit for density detection for each image carrier in each imaging unit leads to a cost increase. Further, a photosensitive material having a diameter as small as 40 millimeters or less has been recently used, in order to decrease a size of a whole system. In a system using such a small-diameter photosensitive material, however, there is no space to arrange the optical detecting unit for density detection around the photosensitive material. Therefore, such a method is adopted that a toner patch for density detection formed on the image carrier in the respective imaging units is transferred onto the transfer belt, and these density patches are detected by a sensor arranged opposite to the transfer belt.
However, when a density patch for each color is formed on the transfer belt, problems described below occur with the lapse of time. That is, as for the transfer belt and the intermediate transfer belt, a belt cannot be easily replaced by users, and since the cost of the whole belt unit is high, a longer service life is often set as compared with that of the photosensitive unit and the development unit. However, since the transfer belt is brought into contact with the transfer paper at all times, both in the tandem-type direct transfer method in which the transfer belt directly transfers a toner image on an image carrier onto paper carried on the belt, and in the intermediate transfer method in which the respective color toner images formed on the intermediate transfer belt are collectively transferred onto paper, the surface of the transfer belt becomes rough due to paper dust with the lapse of time.
When the surface of the transfer belt or the intermediate transfer belt becomes rough with the lapse of time, if detection is attempted by a density detection sensor of a regular reflection output type as illustrated in FIG. 14, as the surface roughness in the background of the transfer belt deteriorates, the sensor output difference between the background and a low transfer patch decreases. Therefore, in the case of a color toner, if the surface roughness Rz (10-points average roughness) of the transfer belt becomes equal to or lower than 1.0 micrometers, only a transfer of 0.2 mg/cm2 at maximum can be detected with respect to a transfer target value in a solid part, 0.6 mg/cm2 (for the Bk toner, detection is possible up to 0.4 mg/cm2 at maximum).
FIGS. 3 and 4 are graphs illustrating the relation between the amount of toner transfer and the sensor output (regular reflection light) when the surface roughness of the transfer belt is different (3 types), respectively in the black toner and the color toners. From these graphs, it is seen that as the surface roughness in the background of the transfer belt deteriorates (the value of Rz increases), a change in the output when the amount of toner transfer changed is small (a sensor output difference due to the transfer decreases).
In the above explanation and FIG. 4, in the case of a color toner, the reason why the maximum value of transfer detectable by the regular reflection output is set to 0.2 mg/cm2 when Rz is equal to or larger than 1.0 micrometer (marks ∘ and ⋄ in FIG. 4) is that the range in which transfer detection by the regular reflection output is possible is an area where the regular reflection output with respect to the transfer indicates a monotonous decrease, that is, a transfer area from a low density pattern portion to a pattern portion giving a minimum value in the output voltage in order in the continuous gradation pattern.
The reason why the regular reflection output changes from a monotonous decrease to a monotonous increase at a certain transfer (0.2 to 0.4 mg/cm2) or more is that as illustrated in FIG. 31, in color toners, the diffuse reflection light from the toner increases with an increase in the transfer, and the diffuse reflection components enter into the regular reflection photodetector.
FIG. 31 is a diagram illustrating the situation in which a belt surface and a solid part of the color toner (cyan here) are detected by the P sensor, wherein in the case of reflection on the belt surface (left side in the figure), diffuse reflection light is small, and hence the influence on the regular reflection photodetector 252 is small. On the other hand, in the case of a cyan solid part (right side in the figure), the diffuse reflection light increases, and is detected by the regular reflection photodetector 252, together with the regular reflection light.
When a transfer belt applied with surface coating is used (that is, in the tandem-type direct transfer method in which toner images are directly transferred from the respective image carriers arranged in tandem onto recording medium supported and carried on the transfer belt, when high-resistance coating is applied on the belt surface in order to obtain a necessary function of electrostatically attracting the paper onto the transfer belt reliably, or in the intermediate transfer belt method, when high-resistance coating is applied on the belt surface in order to prevent dust on superposed images formed on the belt), the surface characteristics expressed by roughness and gloss level certainly deteriorate due to coating as compared with the surface of a base layer of a single-element substance of resin, in addition to deterioration due to wear. Therefore, there is a problem in that the margin with respect to the service life decreases.
On the other hand, if a diffuse reflection sensor as illustrated in FIG. 15 is used, sensor output characteristics of monotonously increasing with an increase in the amount of the color toner transfer, as illustrated in the graph of FIG. 5, can be obtained without being affected by the belt surface characteristics expressed by the roughness and gloss level on the belt surface. As a result, transfer detection is possible up to a high transfer area. On the contrary, there are problems in that, as illustrated in the graph of FIG. 6, this type of sensor is difficult to handle because sensitivity adjustment cannot be performed due to a difference in sensitivity of the sensor in the belt background, since the sensor output in the background of the transfer belt is substantially zero, and on a black transfer belt in which carbon is dispersed such as the transfer belt, detection itself is not possible, since the sensor sensitivity against an increase in transfer is zero with respect to the black (Bk) toner having substantially the same absorption property as the transfer belt.
When sensitivity adjustment of the optical sensor of the diffuse reflection light detection type is performed, adjustment is required so that the output at a transfer (equivalent), where the sensor output is sufficiently high, becomes a predetermined value (as a specific example, for example, the sensor sensitivity is adjusted so that an output voltage value with respect to a certain reference white board inspection plate becomes a predetermined value at the time of factory shipment). However, even if such adjustment is performed initially, the age-based sensitivity changes due to the temperature characteristics of the sensor or deterioration of the light emitting diode, thereby causing a problem in that age-based guarantee is difficult.
Therefore, a method in which a sensor of a type using both regular reflection output and diffuse reflection output is used, so as to detect the black toner by the regular reflection light and color toners by the diffuse reflection light is desired. However, as described above, with regard to the color toners, the diffuse reflection output type sensor is difficult to handle because the sensitivity cannot be adjusted.
In the color image forming apparatus, since a change in the image density leads to a change in hue, it is important to accurately detect the transfer on the density detection pattern to perform density control, in order to stabilize the image density.
The image density to be stabilized here indicates the “image density of the output image”. Therefore, while the conventional monochrome image forming apparatus performs density detection on the photosensitive material, in the color image forming apparatus, it is desired to perform density detection on the transfer belt immediately before being transferred onto the paper. Further, since the purpose of the image density control is to perform control so that the maximum target transfer becomes an aimed value, it is desired that accurate detection up to a high transfer area is possible.
However, in the conventional detection method, it is difficult to detect the transfer stably and accurately at all times over the whole transfer area.